The present invention provides a method for preparing high concentrations of dye molecules in polymers. More specifically, the present invention allows production of optically transparent polymeric devices, such as optical limiters, that become opaque at high light intensities. The high concentration of the nonlinear dye enables high performance optical limiting.
Colorants are a common group of polymer additives. They can be organic dyes or inorganic dyes. The nature of the colorant affects the optical properties of materials. Initially, colorless material, such as polycarbonates and those based on acrylic or styrenic polymers, can either be made translucent, opaque, colored or colorless.
In general, colorants are added to molten polymers as highly concentrated solid solutions of dyed pellets. These pellets are formed by adding a highly concentrated colorant, which is miscible with the desired host polymer, to a dye-receptive polymer. However, colored polymers formed in this way can create a three-phase system, making it more difficult to determine the relevant properties. Alternatively, the colorant may be added directly to the polymer during melt processing. In both methods, the formed colored polymers appear to have a uniform coloration at the macroscopic level. But, the dye molecules are often dispersed inhomogeneously in the polymer matrix. While this inhomogeneous dispersion may be acceptable where simple coloration is desired, it is not acceptable for certain new applications that require both high dye concentrations and the control of chromophore dispersion.
In the case of all-optical limiters the choice of both dye and polymer host is critical in formation of the polymer/dye solid solution. The dye molecule must be compatible with the host plastic, processing method and final use of the finished material, while not imparting any non-desirable rheological properties on the system. In addition, sufficient quantities of the dye have to be present for acceptable device properties, as well as formation of a molecular solution of the dye in the polymer host. The requirement of high dye concentration combined with the need for homogeneous dye distribution in the polymer matrix has necessitated the development of new methods of adding dyes to polymers to both promote molecular dispersion and limit the residence time of the polymer/dye system in conventional melt processing apparatus.
Standard industrial preparatory procedures are unable to provide the necessary concentrations or control over the aggregation of dye molecules. Currently, dye/polymer solutions can be prepared by melt processing; incorporating pigments into the polymer backbone via synthetic procedures; spray coating polymer surfaces with dye solutions; mixing the polymer and dye solution followed by spray coating; and adding highly concentrated colored pellets to conventional processing. However, each of these methods has its drawbacks.
In one instance, melt processing can involve loss of up to 75% of the dye during the current process. For example, a polymer and dye system using 200 g of polymer and 20 g of dye (10% weight/weight) resulted in a solid solution of only 2% weight/weight, which is significant if the dye is a high cost specialty dye. Conventional (proprietary) methods used to add coloration to bulk poly(carbonate) often work poorly with large dye molecules. In addition, prolonged melt processing can cause degradation of the dye and/or polymer.
Incorporating pigments via synthetic procedures also presents problems in that it requires new synthetic procedures to be developed for each polymer and dye system. The net result is the development process is slowed and can become prohibitively expensive.
In addition, spray coating polymer surfaces with dye solutions can be problematic. Proprietary methods used to add dye to the surface of poly(carbonate) have been successful with the phthalocyanine family of dyes. However, the resulting coating is difficult to grow to thicknesses greater than about 1 μm.
Even solution mixing followed by casting/spray/spin coating does not reliably yield molecular solutions. While spray coating is able to form ultra thin films, it can only reliably form films ≈1 μm thick. This method is not amenable to layered structures or films thicker than 50 μm required for phthalocyanine-based optical limiting devices.
Furthermore, adding high concentration colored pellets to conventional processing presents a challenge. The pellets are almost certainly not commercially available with the specialty dyes being used in photonic applications. These pellets may be formulated using another dye receptive polymer, which is miscible with the desired polymer host, resulting in a three-phase system. Such a result could cause changes in the chemical and physical (including photophysical) properties of the system.
Consequently, it would be desirable to have high dye concentrations of dye molecules in polymers as monomeric molecular solutions for the production of working optical limiting devices or any photonics application requiring these standards. It would also be desirable to have stringent control of dye distribution in polymers.